A video display operates by displaying video images in response to a composite video signal received from a video source. The composite video signal comprises both luminance (luma) and chrominance (chroma) information. The luma portion contains the brightness, or black-and-white information for a given series of pixels, and the chroma portion contains the color information for the series of pixels. There are several standards for video signals commonly in use today, one being the National Television Standards Committee (NTSC), which is used in the United States and Japan. In NTSC, the chroma portion of a composite signal shifts in phase by one hundred eighty degrees between the corresponding pixels on consecutive lines and frames. Another standard for video signals is Phase Alternating Lines (PAL), which is used in Europe. In PAL, the chroma component of the composite signal shifts in phase by approximately 270 degrees from line to line.
It will be appreciated chroma information within a composite video signal can be carried within the same frequency band as the luma information. After the horizontal sync pulse, a reference sine wave are added as a color burst. Following the color burst, a phase shift in the chrominance signal indicates the color to display. The amplitude of the signal determines the saturation. The chrominance signal is separated out of the composite video signal and decodes it, along with the normal intensity signal, to determine the color content of the input signal.
When received by video display, the composite video signal is typically input to a video decoder, which separates the luma and chroma information and applies gain and offset factors to them in order to map them to a standard output range. A problem arises from portions of the luma signal that intrude into or near a frequency band associated with the chroma subcarrier. The intruding portion of the luma signal can be demodulated and output from the video decoder along with the chroma signal, resulting in a display of false color. This problem can be seen around sharp contours in text characters and diagonal lines. Another problem that can occur is dot crawl, which occurs when chroma information intrudes into the luma portion of the frequency band. For example, dot crawl can occur where two different colors are adjacent to one another horizontally.